Corrosion inhibitor



pr 21, i953 G. H. RGHRBACK Erm. 2,635,997

CORROSION INHIBITOR 2 SHEETS-SHEET l Filed May 21, 1951 IEM INVENTORS G/Lso/v H. ROHRBA CK ow/.TE' M. MccLoUD w/L/ ARD R. $6077 April 2, 1953 G. H. RoHRBAcK .E-r AL. 2,635,997

CORROSION INHIBITOR GILSON H. ROHRBACK DW/TE W. MCCLOUD W/LLARD R. SCOTT Patented Apr. 21, 1953 UNITED STATES PATENT GFFICE CORROSION INHIBITOR ration, San Francisco, Calif., a corporation `of Delaware Application May 21, 1951, Serial No. 227,351

7 Claims.

This invention relates to a corrosion inhibitor for use in inhibiting the corrosion of ferrous metal piping and tubing in producing oil wells and in pipelines transporting produced crude oil. More particularly, it relates to a solid arsenous corrosion inhibitor.

The corrosion of ferrous metal tubing and piping in producing oil wells is now and has for some years been a serious operating problem confronting the petroleum producer. Corrosion of tubing and piping necessitates frequent interruptions of production to pull corroded tubing and piping from the well and replace them with new material. AThe essential features of a corrosive environment very commonly encountered in producing oil wells include a ferrous metal in contact with oil, brine, and gas containing carbon dioxide, gas liquid interfaces in contact with the metal, and areas of turbulent liquid ow in contact with the metal surfaces. The metal in this environment is corroded away by direct attack of carbonio acid which is greatly accelerated by electrochemical phenomena arising out of the contact of the phase interfaces with the metal and out of the contact of turbulent flowing liquid with the metal surface. Substantially all of the acidity of the production stream stems from the presence of carbon dioxide. The acidity is quite low, the pH not being lower than 3 and being ordinarily in the range 4 to 6. When a ferrous metal is exposed to the production effluent under quiescent conditions where there is no movement on the liquid or of free gas bubbles through the liquid, the corrosion rate observed is usually less than about 25% of that observed under actual p-roducing conditions which include turbulent motion of the liquid in contact with the metal and the 'presence of minute gas bubbles which contact the metal. In laboratory apparatus in which the separate influence of the three corrosive factors was studied, i. e., direct acid attack, electrochemical attack attributable to turbulent liquid flow, and electrochemical attack attributable'to the presence of finely-divided gas bubbles which contact the metal surfaces, it was found that the relative rate of corrosion due to direct acid attack was 4 units, and that the relative rate of'corrosion when either the condition of liquid turbulence or finely-divided gas bubbles moving through the liquid was superimposed on the presence" vof the acid medium, the relative corrosion rate "immediately increased to 14 units. From these observations it is clear that the cory 'mwould'not be 'solved by the yemploy n df ir vibitor which would eliminate only the corrosive action due to direct acid attack and that the corrosion problem here presented can be solved only if the additional lectro'- chemical corrosiony attributable largely tophfysical conditions is' markedly reduced. The crrosion inhibitor of this invention is especially well adapted to minimizing the corrosion of ferrous metals in the environment and by the mechanisms described.

A variety of physical setups is found in producing oil wells which determine the manner of introduction of the corrosion inhibitor into the well. Some wells have open annuli and a corrosion inhibiting solution can be lubricated into the well through the open annulusi Some wells have packed-off annuli and the corrosion inhibitor must be introduced through the production tubing by pumping an inhibitor solution to the well bottom and permitting it to ow upward with the production stream or by dropping a solid corrosion inhibitor through the tubing to the well bottom and producing the well. In condensate wells, Where there is no liquid production stream, the corrosion inhibitor to be effective must in some manner be applied to the interior surface of the tubing without the aid of a liquid production stream to carry and spread it.

A solid corrosion inhibitor which can be placed at or near the well bottom by dropping it through the well tubing can be utilized effectively in all types of wells if the solid inhibitor has the following properties: First, it must be an effective inhibitor at low concentrations. Only relatively small amounts of an inhibitor can be introduced into a Well by dropping the solid inhibitor through the tubing. Some commonly used inhibiting chemicals such as dichromate and caustic soda will inhibit satisfactorily if used in large amounts, but these materials are not suitable for injection into the well in solid form since the maintenance of the desired concentration requires very -frequent interruption of the production for the introduction Qf further amounts of the inhibitor. A suitable chemical inhii'btor for injection into the well through the tubing in solid form would he one that provides protection against corrosion at concentrations'of about one to rive parts per million in the produced water; second, an 'efffec tive 'solid corro'sion'inhibitor should `have a high persistency in its inhibiting effect. To be practical',l the injections of a "solid inhibitor should not be made more' often than once in twentyfour hours, 'and preferably once in several days, or once-a eek.' tencyofthe'inhibitor i himetrl Surface 3 of the tubing is highly desirable in solid inhibitor injection. Third, a successful solid corrosion inhibitor should be highly soluble in well water. 'I'his is particularly desirable if the inhibitor is to be used in a condensate well. Relativelyinsoluble or highly insolublematerials could easily cause plugging of the tubing. Fourth, a solid corrosion inhibitor should have a high density so thatthev solid inhibitor will fall through the production stream while the well is being produced. If the solid inhibitor has this property, the production need be interrupted for only the few minutes necessary to introduce the solid into the tubing. Fifth, the solid inhibitor, if it is to be effective in a condensate well, should have a low melting or softening temperature. Condensate wells produce little or no formation Water and much of a water-soluble chemical that falls below the condensate zone will not be returned with theV production. If the solid inhibitor softens or melts at temperatures from about 130 to 180 F., the solid will be spread on the hot tubing wall as it falls in the well and will give good distribtuion of the inhibitor over the interior of the tubing.

Arsenous compounds, particularly the alkali metal arsenites or slurries of arsenous oxide with alkali metal hydroxides and Water, meet the above-described requirements with respect to effectiveness at low concentration, persistence, and A solubility admirably. ,Particular arsenous compositions hereinafter described meet not only the first three requirements, but also the requirements with respect to density and to softenin point in a remarkable'manner.

It has been found'that a hard, dense, homogeneous solid corrosion inhibitor can be prepared by intimately mixing arsenous oxide with an aqueous solution of sodium hydroxide and cooling the resultant mixture to remove exothermic heat of reaction. The mixture Vcontains from 1.5 to 6.0v parts by weight of water, preferably 1.5 to 3.5 parts by Weight of Water to 15 parts by weight of sodium hydroxide and arsenous oxide together and the ratio of arsenous oxide to sodium hydroxide in the mixture is in the range 1:2 to 1:14, and preferably in thc range 1:2 to 1:4.

Compositions containing these ingredients in these proportions are extremely hard and durable at ordinary temperatures, have a density above about 2.0 grams per cubic centimeter and are thermoplastic in the sense that they become soft at temperatures in the range about 150 to 170 F. and finally become liquid at temperatures above about 200 F.

The properties of numerous compositions consisting essentially of arsenous oxide, sodium hydroxide and water have been studied. Observations of these compositions are reported in tables of data hereinafter and some of the observations are graphically represented in the appended drawings of which Figure 1 is a graphical representation of the variation in hardness of the conipositions as the proportions of arsenous oxide and sodium hydroxide are varied while holding the water content constant, and Figure 2 is a graphical representation of the variation of the properties of the compositions as the proportions of the three components, arsenous oxide, sodium hydroxide and Water, are varied.

The results of a number of experiments in which the setting characteristics of varying mixtures of arsenous oxide, sodium hydroxide and water are set forth in the following Table I.

Small batches of these chemicalsV were `mixed. using different proportions of the materials in each mixture. The mixtures were most readily made by dissolving sodium hydroxide in water and then adding arsenous oxide to the solution with rapid stirring. A considerable heat of reaction Was evolved upon theaddition of the arsenous oxide, the temperature rising to about F. and the mixtures becoming highly fluid and homogeneous. After standing at room temperature for 24 hours the mixtures were examined for hardness and rated as hard, soft or liquid on an arbitrary scale. The rating hard Was applied where the material could not be distorted by manual pressure, and the rating "soft was applied where the sample could be deformed by manual pressure.

Non-homogeneous mixtures-2 phases form-aqueous NaOH and solid-hardness data is for solid.

From the table it can be seen that those mixtures in which the proportion of water ranged from 1.5 to 3.5 parts by weight, hardness was encountered at two ratios of sodium hydroxide to arsenous oxide, that is, at the ratio of 7:8 Where these materials were present in approximately equal amounts and at the ratio of 11:4 Where the .amount of sodium hydroxide considerably exceeded the amount of arsenous oxide.

In conducting the experiment summarized in Table I, it was found that approximately 11/2 parts by weight of Water to a, total of 15 parts by Weight of sodium hydroxide and arsenous oxide together appeared to be about the minimum amount of water with which a homogeneous mixture could be prepared. `Somewhat smaller amounts of water can probably be employed and a satisfactory degree of homogeneity achieved with more vigorous mixing.

A further series of mixtures was prepared to determine more accurately the eiect of varying the proportion of arsenous oxide and sodium hydroxide in the mixtures. The results of these tests are graphically represented in Figure 1 of the appended drawings. In all of the mixtures whose properties are shown in Figure 1, two parts by weight of water were mixed with a total of 15- parts by weight of arsenous oxide and sodium hydroxide. After the mixtures were prepared they were permitted to stand at room temperature until cool and then were tested for hardness and assigned an arbitrary hardness classification by means of a simple probe test. A pointed metal rod, 1/8 inch in diameter, was pressed into the surface of each mixture and then removed. The hardness classifications used are as follows:

Paste: I'he rod entered the mixture easily and the hole tended to ll when the rod was removed. Clay: The rod entered the mixture easily' butthe" hole maintained its shape when the rod was removed. Hard: The rod could be forced into the surface but not much beyond the sharp point. Very hard: The rod could not be forced into the surface without chipping the solid.

Hardness classications were made by two different observers independently and the results were plotted as shown in Figure 1.

Figure 1 shows a rapid change in the hardness ide, and with one of these compositions the proportion of water was varied. Test tubes vccm-f taining the samples were placed in an oil bath and allowed to stand at each indicated temperae ture for a period of 30 minutes which was ascere tained to be long enough to bring the contents of the test tube to oil bath temperature. At the rst indication o f softening the samples were classified as slightly soft, at ,definite softening they were classied as sofa and if they became fluid they were lassiiled as liquid TABLE II Composition Tempergme, .e EA

Hfdtlg A5203, NaOH' f al'ts by I rgaresby Parts byy Wt- 90 95 11o 13o 14o 159 41ro ieg@ 2go Wi, Wt.

sig er 2 u u sis s S. Narrow Central Range 8% 6% 2% E H SLS S ,8% 6% 3% H H .BIAS vS, L -i---e- 5' 19 2 H H H .$1.8 v S iL L L ,L High NaOH Yoncentration 4 11 2 E H H H H S 'S S VS Bange 3 12 2% H 1H 'H ,H H SL-S -f :S ;.L .L 2 1a 2% E H H H E` 51.5 S S- S 1 14 2% H H H E' H srs L L LA of the mixtures for small changes in the proportion of arsenous oxide to sodium hydroxide. Two ranges of hard set mixtures are shown in Figure 1. To produce the rst series of hard set mixtures the ratio of arsenous oxide to sodium hydroxide is in the range 1:1 to 2:1, the optimum hardness being attained at arsenous oxide to sodium hydroxide ratios in the range 1.1:1 to 1.5:1.

A second series of hard setting mixtures is shown by Figure 1 to be formed when the ratio of arsenous oxide 4to sodium hydroxide is from 1:2 to 1:14. These mixtures containing high ratios of sodium hydroxide to arsenous oxide, i. e., ratios from 2:1 to 14:1, set to form very hard solids when the amount of water employed in the mixture ranged from 1.5 to 6.0 parts by weight with par-ts by weight of sodium hydroxide and arsenous oxide together. If more than 6.0 parts by weight of water were used, the mixture remained soft when cool.

'I'he second series of hard setting mixtures of arsenous oxide, sodium hydroxide and water are prepared by dissolving the indicated amount of sodium hydroxide in water and then dissolving the arsenous oxide in the aqueous sodium hydroxde solution. The mixture is then cooled to room temperature and upon cooling it sets, forming a hard solid having a density above about 2.0. The hard solid softens at about 150 F. These mixtures containing arsenous oxide and sodium hydroxide such that the AszOa/NaOI-I ratio is in the krange 1:2 to 1:14, and containing from 1.5 to 6.0 parts by weight of water to 15 parts by weight of arsenous oxide and sodium hydroxide together, are effective corrosion inhibitors against corrosive attack of ferrous metals by oil Well production streams. The hard compositions in which the ratio of arsenous oxide to sodium hydroxide is 1:4 or higher, and in which the water content does not exceed about 3.5 parts by weight to 15 parts by weight of arsenous oxide and ysodium hydroxide together, are preferred by reason of their greater densi-ty and higher arsenic content.

Data showing the effect of temperature in the two types of hard setting compositions shown in Figure 1 are summarized in the following Table II. The compositions tested had ve different prepprtions of .arsenous oxide to sodium .liyiluuiE CFI The data summarized in above Table -II shows that the softening temperature of the two hard setting ranges are quite different. The hard setting, high arsenous oxide compositions sofwtened at about F., and water content within the upper and lower limits for good mixing does not affect this temperature appreciably. Mixtures in the low arsenous oxide, hard setting range soften at about 1509 F. with the exception of the 5 to 10 mixture which is at 'the left-hand boundary of this hard vsetting range, as shown in Figure 1, and which softens at about F.

'I'he setting characteristics of diierent mixtures of arsenous oxide, sodium hydroxide and water are generally summarized in graphical form in Figure 2 of the drawings. The two clear areas in Figure 2 indicate hard homogeneous mixtures of arsenous oxide, sodium hydroxide and water. All of the larger of these two areas on the right-hand side of the drawing represent operable mixtures formed from 1.5 to 6.0 parts of water with 15 parts of sodium hydroxide and arsenous oxide together and having the sodium hydroxide and arsenous oxide present in ratios in the range 2:1 to 14:1. However, preferred mixtures having a somewhat higher density, lower softening point, and considerably higher content of the active arsenous component of the corrosion inhibitor are represented by the smaller portion of the clear area enclosed lby the broken line. These compositions are prepared `as indicated above by stirring arsenous oxide in an amount within the range indicated in the figure into an aqueous solution of sodium hydroxide containing these two ingredients in the proportions indicated in the figure. The resulting mixture is very hot and highly fluid and can be poured into molds 0f any .desired shape and allowed to solidify yin the molds. The resultant solids are macroscopically homogeneous and have a density above about 2 grams per cubic centimeter and a softening point around F. These compositions can be heated until they become liquid and resolidified without affecting their physical properties o r homogeneity. This characteristi makes .it possible to prepare large solid blocks of a composition which may be transported some distance from the point of manufacture, if desired, and fthere melted and cast into any desired denn.

invention are ordinarily cast in the form of a cylindrical stick from 1 to 3 feet in length and having a diameter from 1 to 2 inches,l that is, a diameter generally adapted to permit the fall of the stick through conventional well tubing. The sticks ordinarily weigh from 2 to 6 pounds and have a density above about 2 grams per cubic centimeter. Their mass and density are such that they fall through the production stream of the average producing well so that the production need be interrupted only for the few minutes which are required to introduce the stick into the well tubing. The introduction of such a stick everyy 2 to. 6 days will ordinarily bring the corrosion rate of a well down t an acceptably low level and'maintai'n it there.

Since arsenical compounds are poisonous, the

sticks are desirably wrapped or coated with some non-toxic material in the interest of safe handling primarily. The Wrapping or coating may into the well, or may be removable at the time of the injection.

Metals having a standard oxidation reduction potential above 0.5and below 2.5 volts, such as magnesium, aluminum, zinc, and their alloys, form excellent functional coatings for the inhibitor. Corrosion inhibiting cartridges can be prepared by pouring the molten corrosion inhibitor into these metallic tubes and permitting it to solidify there. The bottom of the tube is closedto prevent the dissolving of the inhibitor from that end of the tube in the well. The bottom then can be closed off by capping with a metallic cap, or crimping, or sealing with a high melting point asphalt. If an asphalt plug is used in the bottom of the metal tube, the. asphalt is selected to have a melting point above the bottom hole temperature of the well. Such a plug is removed by gradual dissolving of the asphalt in the oil phase. This process requires a sumcient period oiv time that most cf the inhibitor goes into solution from the top of the tube. The top of the tube isv ordinarily sealed with a high melting point wax, for example, a wax melting from about 125 F. to 150 F. The metallic coating is an integral part of the inhibitor cartridge as it is dropped into the well and the metal is functional in its action in the Well and not merely a' container for the inhibitor. Magnesium, for example, slowly dissolves in dilute carbonio acid contained in the well water and, in dissolving,

neutralizes a p-art of the acid and, in addition, metals such as magnesium, zinc, aluminum, and

rapidly dissolved, placing the inhibitorcornposition in contact with the production stream.

The inhibitor sticks may also be protected by an outer container which is removed prior to the introduction of the inhibitor stick into the well. The hot, soft corrosion inhibitor is poured into paper cylinders which would maintain their shapes. when the'temperature ofthe inhibits-rv reaches or exceeds its softening point' during ship- 8 ment or storage. The paper tubing is peeled away from the stick before dropping it into the well tubing. It has been found that if the hot corrosion inhibitor mixture is poured into untreated paper tubes, some diiiculty is experienced in attempting to remove the paper from the hardened stick. However, if the paper is greased with a fairly heavy grease, or covered with a water impervious coating such as shellac or plastic, prior to the introduction of the molten inhibitor composition into the paper tube, the paper may easily be removed from the solidified stick.

We claim:

1. A hard, dense macroscopically homogeneous solid prepared by intimately mixing arsenous oxide with 4an aqueous solution of sodium hydroxide and cooling the resultant mixture to remove exothermic'heat of reaction, said mixture being formed from 1.5 to 6.0 parts by weight of water to 15 parts by weight of sodium hydroxide and arsenous oxide together and containing arsenous oxide and sodium hydroxide, respectively, at a weight ratio in the range 1:2 tc 1 :14.

2. A hard, dense, homogeneous solid prepared by intimately mixing arsenous oxide with an aqueous solution of sodium hydroxide and cooling the resultant mixture to remove exothermi-c heat of reaction, said mixture being formed from 1.5 i to 3.5 parts by weight of water to 15 partslcy` weight of sodium hydroxide and arsenous oxide together and containing arsenous oxide and sodium hydroxide, respectively, at a weight ratiol in the range 1 :2 to 1:4.

3. A hard, dense solid comprising the reaction product of arsenous oxide, sodium hydroxide and water, the ratio of arsenous oxide toV sodium hydroxide being in the range 1:2 to 1:14 and the Water content being at least one part byweight and not more than 6.0 parts by weight to eachl 15 parts by weight of arsenous oxide plus sodium hydroxide.

4. A hard, dense solid comprising an intimate mixture of sodium hydroxide and arsenous oxide with water, said mixture being formed from 1.5 to 3.5 parts by weight of water and 15 parts by Weight of sodium hydroxide and arsenous oxide together and having arsenous oxide and sodium hydroxide present at a weight ratio in the range 5. A hard, dense solid comprising an intimate f mixture of sodium hydroxide and arsenous oxide with Water, said mixture being formed from 1.5 to 3.5 parts by weight of water and 15 parts by weight of sodium hydroxide and arsenous oxide together and having arsenous oxide and sodiumhydroxide present at a weight ratio in the range 1:2 to 1:4.

6. A hard, dense thermoplastic solid formed by slurrying arsenous oxide in an aqueous sodium hydroxide solution and cooling the resulting .ducing oil well delivering a production stream.

comprising crude oil, brine, and carbon dioxide gas whlch comprises periodically interrupting the production, dropping a solid corrosion inhibitor,. formed by intimately mixing arsenous oxide and sodium hydroxidewithwater, said mixture contaimng from 1.5 to 6.0 Vparts'by weight'of water References Cited in the file of this patent UNITED STATES PATENTS Number Name Date Sherbino Aug. 9, 1927 Grebe Sept. 13, 1932 Karr Mar. 12, 1946 Cross Mar. 27, 1951 

1. A HARD, DENSE MACROSCOPICALLY HOMOGENEOUS SOLID PREPARED BY INTIMATELY MIXING ARSENOUS OXIDE WITH AN AQUEOUS SOLUTION OF SODIUM HYDROXIDE AND COOLING THE RESULTANT MIXTURE TO REMOVE EXOTHERMIC HEAT OF REACTION, SAID MIXTURE BEING FORMED FROM 1.5 TO 6.0 PARTS BY WEIGHT OF 